Re: [PATCH v8 01/15] x86/boot: Place kernel_info at a fixed offset

Ard Biesheuvel
2024-02-15T07:56:39Z

On Wed, 14 Feb 2024 at 23:31, Ross Philipson <ross.philipson@oracle.com> wrote:
>
> From: Arvind Sankar <nivedita@alum.mit.edu>
>
> There are use cases for storing the offset of a symbol in kernel_info.
> For example, the trenchboot series [0] needs to store the offset of the
> Measured Launch Environment header in kernel_info.
>

Why? Is this information consumed by the bootloader?

I'd like to get away from x86 specific hacks for boot code and boot
images, so I would like to explore if we can avoid kernel_info, or at
least expose it in a generic way. We might just add a 32-bit offset
somewhere in the first 64 bytes of the bootable image: this could
co-exist with EFI bootable images, and can be implemented on arm64,
RISC-V and LoongArch as well.

> Since commit (note: commit ID from tip/master)
>
> commit 527afc212231 ("x86/boot: Check that there are no run-time relocations")
>
> run-time relocations are not allowed in the compressed kernel, so simply
> using the symbol in kernel_info, as
>
> .long symbol
>
> will cause a linker error because this is not position-independent.
>
> With kernel_info being a separate object file and in a different section
> from startup_32, there is no way to calculate the offset of a symbol
> from the start of the image in a position-independent way.
>
> To enable such use cases, put kernel_info into its own section which is
> placed at a predetermined offset (KERNEL_INFO_OFFSET) via the linker
> script. This will allow calculating the symbol offset in a
> position-independent way, by adding the offset from the start of
> kernel_info to KERNEL_INFO_OFFSET.
>
> Ensure that kernel_info is aligned, and use the SYM_DATA.* macros
> instead of bare labels. This stores the size of the kernel_info
> structure in the ELF symbol table.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Cc: Ross Philipson <ross.philipson@oracle.com>
> Signed-off-by: Ross Philipson <ross.philipson@oracle.com>
> ---
> arch/x86/boot/compressed/kernel_info.S | 19 +++++++++++++++----
> arch/x86/boot/compressed/kernel_info.h | 12 ++++++++++++
> arch/x86/boot/compressed/vmlinux.lds.S | 6 ++++++
> 3 files changed, 33 insertions(+), 4 deletions(-)
> create mode 100644 arch/x86/boot/compressed/kernel_info.h
>
> diff --git a/arch/x86/boot/compressed/kernel_info.S b/arch/x86/boot/compressed/kernel_info.S
> index f818ee8fba38..c18f07181dd5 100644
> --- a/arch/x86/boot/compressed/kernel_info.S
> +++ b/arch/x86/boot/compressed/kernel_info.S
> @@ -1,12 +1,23 @@
> /* SPDX-License-Identifier: GPL-2.0 */
>
> +#include <linux/linkage.h>
> #include <asm/bootparam.h>
> +#include "kernel_info.h"
>
> - .section ".rodata.kernel_info", "a"
> +/*
> + * If a field needs to hold the offset of a symbol from the start
> + * of the image, use the macro below, eg
> + * .long rva(symbol)
> + * This will avoid creating run-time relocations, which are not
> + * allowed in the compressed kernel.
> + */
> +
> +#define rva(X) (((X) - kernel_info) + KERNEL_INFO_OFFSET)
>
> - .global kernel_info
> + .section ".rodata.kernel_info", "a"
>
> -kernel_info:
> + .balign 16
> +SYM_DATA_START(kernel_info)
> /* Header, Linux top (structure). */
> .ascii "LToP"
> /* Size. */
> @@ -19,4 +30,4 @@ kernel_info:
>
> kernel_info_var_len_data:
> /* Empty for time being... */
> -kernel_info_end:
> +SYM_DATA_END_LABEL(kernel_info, SYM_L_LOCAL, kernel_info_end)
> diff --git a/arch/x86/boot/compressed/kernel_info.h b/arch/x86/boot/compressed/kernel_info.h
> new file mode 100644
> index 000000000000..c127f84aec63
> --- /dev/null
> +++ b/arch/x86/boot/compressed/kernel_info.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef BOOT_COMPRESSED_KERNEL_INFO_H
> +#define BOOT_COMPRESSED_KERNEL_INFO_H
> +
> +#ifdef CONFIG_X86_64
> +#define KERNEL_INFO_OFFSET 0x500
> +#else /* 32-bit */
> +#define KERNEL_INFO_OFFSET 0x100
> +#endif
> +
> +#endif /* BOOT_COMPRESSED_KERNEL_INFO_H */
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 083ec6d7722a..718c52f3f1e6 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -7,6 +7,7 @@ OUTPUT_FORMAT(CONFIG_OUTPUT_FORMAT)
>
> #include <asm/cache.h>
> #include <asm/page_types.h>
> +#include "kernel_info.h"
>
> #ifdef CONFIG_X86_64
> OUTPUT_ARCH(i386:x86-64)
> @@ -27,6 +28,11 @@ SECTIONS
> HEAD_TEXT
> _ehead = . ;
> }
> + .rodata.kernel_info KERNEL_INFO_OFFSET : {
> + *(.rodata.kernel_info)
> + }
> + ASSERT(ABSOLUTE(kernel_info) == KERNEL_INFO_OFFSET, "kernel_info at bad address!")
> +
> .rodata..compressed : {
> *(.rodata..compressed)
> }
> --
> 2.39.3
>